Circuit arrangements embodying electron discharge devices



R. S. WEBLEY Nov. 10, 1959 CIRCUIT ARRANGEMENTS EMBODYING ELECTRONDISCHARGE DEVICES Filed NOV. 1, 1954 men-top B.-5.NebZey g M jigoCIRCUIT ARRANGEMENTS EMBODYING ELEC- TRON DISCHARGE DEVICES ReginaldSidney Webley, Hayes, England, assignorto Electric & Musical IndustriesLimited, Hayes, Middlescx, England, a "company of Great BritainApplication November 1, 1954, Serial No. 466,189

@laims priority, application Great Britain November 6, 1953 4 Claims.(Cl. 315-12) This invention relates to circuit arrangements embodyingelectron discharge devices.

In the British patent specification No. 714,924, there are describedvarious electronic storage tubes in which received signals can beapplied to a charge storage target electrode by a so-called writingoperation and the stored charges can be reproduced a number of times bya socalled reading operation. The reading operation is accomplished byscanning the target electrode with a low velocity electron beam in thepresence of a longitudinal magnetic field and some of the electrons inthe reading beam are modulated by the charges stored on the targetelectrode and pass through the latter to be collected by a collectingelectrode to which is connected a signal impedance across which signalsare developed. Some of the electrons in the reading beam are returned tothe electron gun generating the reading beam where they are collected bythe various electrodes comprising the electron gun such as a limiterelectrode, a cathode shield, or the cathode.

An electron discharge tube suitable for television purposes is alsoknown in which a photo-electrically sensitive target electrode isprovided on to which an optical image is projected to be scanned by alow velocity scanning beam in the presence of a longitudinal magneticfield, the arrangement being such that the target electrode isstabilised at a potential corresponding substantially to that of thecathode of the electron gun which generates the scanning beam. In such adevice electrons which are not required for discharging the targetelectrode are also returned to the electron gun generating the scanningbeam. A further electron discharge device is also known in Which thetarget electrode comprises a photo-conductive layer and such device isalso arranged to be scanned in the presence of a longitudinal magneticfield by a low velocity scanning beam. in this device, electrons whichare not collected by the target electrode are also returned to thecathode of the electron gun. In fact, in many electron discharge deviceswhere an electron beam is arranged to scan a target electrode in thepresence of a longitudinal magnetic field and with a low velocity suchthat the target electrode is caused to have an equilibrium potentialcorresponding substantially to that of the cathode of the electron gungenerating the scanning beam, electrons which are not required todischarge the target electrode are returned to the electron gun.

Many proposals have been made in the past for employing the electronswhich are not required to discharge the target electrode for thegeneration of required signals. In some cases an electrode such as alimiting electrode inthe electron gun has been connected to earththrough a signal resistance so that the current collected by thiselectrode and flowing through the resistance sets up the requiredsignals. In this case, it is found that noise arises due to thefluctuation of the electron current from the cathode of the device whichserves to decrease the signalto-noise ratio.

asiasir Patented Nov. 10, 1959 It is the object of the present inventionto provide an improved circuit arrangement by means of which electronsreturned from the target electrode from a scanning beam can be employedto generate signals but in which the signal-to-noise ratio is improved.

According to the invention there is provided a circuit arrangementincluding an electron discharge device having a target electrode and anelectron gun for generating a scanning beam which is caused to scan saidtarget electrode and in which electrons not collected by the targetelectrode are returned to said electron gun and wherein the electrodesof said gun are interconnected for signal frequencies and have a highimpedance-to-ground for said frequencies and a signal frequency outputis derived from the electrodes of said gun.

In order that the said invention may be clearly understood and readilycarried into effect, it will now be more fully described with referenceto the single figure of the accompanying drawing, which illustrates acircuit arrangement in accordance with the invention and as applied toone of the electronic storage tubes disclosed in the specification ofthe aforesaid patent.

The storage tube shown in the drawing is particularly suitable for radarpurposes, and comprises an evacuated envelope 1 having at one end acathode 2 for generating a beam of electrons, a modulating electrode 3,an anode 4, and an electron collecting electrode 5. In the enlargedcentre portion of the envelope 1 there is provided an electron permeablecharge storage target electrode 6 which comprises a metal mesh or gridhaving, for example, a thousand meshes per linear inch and provided onits side facing the cathode 2 with a layer of insulating material suchas calcium fluoride. Between the electrode 5 and the target electrode 6there is provided a further mesh or grid 7 which may function as anelectrostatic shield. At the other end of the tube there is provided afurther cathode 8 adapted to generate a beam of electrons, a cathodeshield 9, a beam limiting electrode 9a, an anode 10 which extends closeto the target electrode 6, and between the cathode S and the electrode 6there is provided a mesh or grid 11 which functions as an ion trap. Theelectron beam from the cathode 2 is scanned over the surface of thetarget electrode 6 in any suitable manner, as by means of scanning coilsindicated at 12 and the electron beam from the cathode 8 is also scannedover the opposite surface of the storage electrode as by means ofscanning coils 13, the beam from the cathode 8 being caused to scan thetarget electrode at a low ve locity and in an orthogonal manner forwhich purpose the beam from the cathode 8 is caused to scan in thepresence of a longitudinal magnetic field generated by a solenoid coil14.

In operating the tube shown in the drawing, the cathode 2 may bemaintained at a negative potential of 1500 volts and the modulatingelectrode 3 is connected to a source 15 of signals which may, rorexample, be signals derived from radar apparatus. The anode 4 may bemaintained at a positive potential of SO-lOO volts and'the collectingelectrode 5 may be connected to a positive potential of about 35 volts.The grid or mesh 7 may he maintained at the potential of the electrode 5or at a more positive potential with respect thereto as hercinaterreferred to. The cathode 8 may be maintained at zero volts, the cathodeshield at a negative potential of about 30 volts, the li iter 9a at apositive potential of about 600 volts and the anode It and the ion trapmesh 11 at a positive potential of about 400 volts. The conductingmember of the target electrode is maintained at a potential of between 2and +5 volts with respect to the cathode 8. The signs of the potentialsapplied to the various electrodes referred to above and hereinafter aretaken in relation to the potential of the cathode 8. The electron beamfrom the cathode 2 serves to perform the writing operation and theelectron beam from the cathode 8 the reading operation. In order tocause the target electrode to store charges corresponding to signalsfrom the signal source 15, the potential applied to the electrodesassociated with the cathode 2 are such that the beam from the cathode iscaused to impinge on the insulating layer of the target electrode with avelocity greater than that corresponding to the first cross-over pointon the secondary electron emission curve of the insulating material sothat a greater number of secondary electrons are released from theinsulating surface compared with the incident electrons so that theinsulating surface becomes charged positively. The order of positivecharging of said insulating surface may be 2-3 volts with respect to thepotential of the cathode 8. The potential applied to the conductingmember of the target electrode is a few volts positive or negative withrespect to the cathode 8 so. that during a reading operation the numberof electrons of the reading beam from the cathode 8 which actually passthrough the target electrode will depend on the stored charge in thevicinity of the beam and hence the electrons will be modulated by thestored charges. The electrons passing through the target electrode fromcathode '8 are collected by the electrode 5.

The electrons during a reading operation which do not pass through theinterstices in said target electrode are returned to the electron guncomprising the cathode 3 and the electrodes 9 and 9a and the requiredsignals are generated from these electrons. For this purpose theelectrodes 8, 9 and 9a have a high impedance-to-ground for the signalfrequencies and as shown the electrode 9a is connected through, forexample, a 1 megohm resistance 16 to a positive source of potential ofabout 600 volts and the cathode 8 is also connected through a resistance17 of about 1 megohm to a point of negative potential of 150 volts. Theelectrode 9 is also connected through a resistance 18 of about 4.7megohms to a source of negative biassing voltage. Furthermore, theelectrodes 8, 9 and. 9a are inter-connected for producing output signalfrequencies for which purpose these electrodes are intercoupled by meansof condensers 19 and 29. Thus, the electrons which are returned from thetarget electrode 6 and which will be collected, in the main,.by thelimiting electrode 9a and also by the cathode 8 and possibly by theelectrode 9 serve to set up signals or, in other words, the net currentleaving the {cathode 8 serves to generate signals which are fed througha condenser 21 to an amplifier 22 and then to a reproducing tube 23. Itwill be appreciated from the above arrangement that the signal-to-noiseratio is improved since the noise associated with the flow'of currentbetween the cathode and limiter electrode 9a flows in a closed circuitwhich does not contain any signalimpedance. i i

It may be found that when the voltage on the electrode 9 is changed, anundesired change in the potential of the cathode 8 occurs and it isaccordingly preferred to employ 'in the circuit a suitable voltagestabiliser for the cathode potential, such a voltage stabilisercomprising a known type of thermionic valve stabiliser employing, forexample, a pentode valve 24 in which voltage stabilisation is effectedbymeans of negative feedback from the anode to the control electrodethereof. As shown, the cathode 8 is connected to the control electrodeof the valve 24 through a resistance 25 which may have a value of 4.7megohms, the control electrode being connected through a condenser 26 toearth. The screen electrode of said valve is connected to a positivesupply terminal of, say, 300 volts through a resistance 27 and the anodeof the valve is connected through a load resistance 28 of, say, 100,000ohms to said positive supply terminal. The anode of the valve 24 is alsoconnected through a resistance 29 to the control electrode for providinga negative feedback,

the example referred to above is zero. The dotted line 30 indicates ascreening enclosure for the parts contained within said enclosure andthis screening enclosure is connected to earth. The various supply leadsare decoupled to earth by the condensers 31 and resistances 32 and 25 asshown.

If desired, signalsrcan also be generated across a signal resistanceconnected to the electrode 5 and employed in conjunction with thesignals fed to the amplifier 22 via the condenser 21 for application tothe reproducing tube 23, one of the generated signals being suitablyreversed in phase and if necessary equalised in amplitude.

With the arrangement in accordance with the present invention in, orderto erase the charges on the electrode 6 the electrostatic shield 7 canbe maintained at a negative potential with respect to the cathode 8generating the reading beam without cutting-ofi the modulated beam whichpasses through the target electrode so that the electrons of saidmodulated beam return to said target electrode and discharge the latter,so that thereby it is possible to obtain minimum erasure times of lessthan 1 second. With the arrangement according to the invention, thewriting gun anode 4, canbe connected to the collector electrode 5 andboth of these electrodes can be maintained at a positive potential withrespect to the cathode of the reading gun. Furthermore, the electrode 7may be maintained at a positive potential with respect to the collectorelectrode 5.

Although the invention has been particularly described as applied to thestorage tube shown in Figure 1 of British patent specification -No.714,924, it will be appreciated that it can also be applied to the tubeshown in Figure 3 of said patent specification and also to otherelectron discharge devices having a target electrode which is scanned bya low velocity scanning beam in the presence of a longitudinal magneticfield such as the devices referred to in the preamble of thisspecification. In all applications of the invention, the signals set upat the electron gun and the signals generated from the collectorelectrode can be employed in push pull if so desired.

What I claim is: t

1. A circuit arrangement including an electron discharge device having atarget electrode, an electron gun spaced therefrom for generating ascanning beam, said gun including a cathode and a further electrodedisposed near to said cathode and disposed to collect electrons, meansto cause said beam to scan said target electrode with a low velocity,means to cause electrons not collected by the target electrode duringscanning to be returned to said cathode and further electrode togenerate output signals, said last named means including meanssurrounding the space between said gun and target electrode to generatea magnetic field, means interconnecting whereby the valve 24 functionsin well-known manner to said cathode and said further electrode forderiving output signal frequencies, means connected to said last namedmeans to provide a high impedance to ground for said frequencies andmeans connected to said high impedance providing means for deriving asignal frequency output from said cathode and further electrode.

2.. A circuit arrangement including an electron discharge device havinga target electrode, an electron gun spaced therefrom for generating ascanning beam, said gun including a cathode, a cathode shield and afurther electrode disposed near to said cathode and disposed to collectelectrons, means to cause said beam to scan said target electrode with alow velocity, means to cause electrons not collected by the targetelectrode dun'ng scanning to be returned to said cathode, cathode shieldand further electrode to generate output signals, said last named meansincluding means surrounding the space between said gun and targetelectrode to generate a magnetic field, condensers interconnecting saidcathode and said furtherelectrode for deriving output signalfrequencies,-a high impedance connecting said cathode, cathode shieldand said further electrode to points of-suitable potential and means forderiving a signal frequency output from said cathode, cathode shield andsaid further electrode.

3. A circuit arrangement including an electron discharge device havingan electron permeable charge storage target electrode, an electron gunspaced therefrom for generating a scanning beam, said gun including acathode and a further electrode disposed near to said cathode anddisposed to collect electrons, means to cause said beam to scan saidtarget electrode with a low velocity, an electron collecting electrodeto collect electrons which pass through said target electrode, a meshbetween said target electrode and said collecting electrode, means tocause electrons not collected by the target electrode during scanning tobe returned to vsaid cathode and further electrode to generate outputsignals,

said last named means including means surrounding the space between saidgun and target electrode to generate a magnetic field, meansinterconnecting said cathode and said further electrode for derivingoutput signal frequencies, means connected to said last named means topro: vide a high impedance to ground for said frequencies and meansconnected to said high impedance providing means for deriving a signalfrequency output from said cathode and further electrode.

4. A circuit arrangement including an electron discharge device havingan electron permeable charge storage target electrode, an electron gunspaced therefrom for generating a scanning beam, said gun including acathode, a cathode shield, and a further electrode disposed near to saidcathode and disposed to collect electrons, means to cause said beam toscan said target electrode with a low velocity, an electron collectingelectrode to collect electrons which pass through said target electrode,a mesh between said target electrode and said collecting electrode,means to cause electrons not collected by the target electrode duringscanning to be returned to said cathode, cathode shield and furtherelectrode to generate output signals, said last named means includingmeans surrounding the space between said gun and target electrode togenerate a magnetic field, condensers interconnecting said cathode andsaid further electrode for deriving output signal frequencies, a highimpedance connecting said cathode, "cathode shield and said furtherelectrode to points of suitable potential and means for deriving asignal frequency output from said cathode, cathode shield and saidfurther electrode.

References Cited in the file of this patent UNITED STATES PATENTS2,245,364 Riesz et al. June 10, 1941 2,537,250 Weimer Jan. 9, 19512,675,499 Sears Apr. 13, 1954 2,726,353 Wallmark Dec. 6, 1955 2,734,135Reed Feb. 7, 1956

